The invention relates to a process for the thermal pyrolysis of hydrocarbons using an electric furnace. This process is aimed, in particular, at the production of light olefins, and, more particularly, of ethylene and propylene.
A number of patents describe processes and reactors for the implementation of these processes. In particular, the assignee's patent U.S. Pat. No. 4,780,196 can be cited which describes a process for thermal pyrolysis in the presence of water vapour which is known as a steam-cracking process and is used in a multi-channel reactor made of a ceramics material. This process gives good yields of ethylene and propylene. However, the reactor is of a delicate design, the ceramics materials used to make it are relatively expensive, and it is difficult to maintain a constant temperature all along the reaction zone which has adverse effects on the process.
The prior art is illustrated, in particular, by the patents EP-A-323 287, EP-A-457 643, FR-A-1 305 287 and U.S. Pat. No. 1,407,339.
One of the major problems encountered with the implementation of thermal pyrolysis, and, in particular, with the steam-cracking of hydrocarbons is connected with the formation of coke. This formation is largely due to secondary reactions such as the formation of condensed polycyclic aromatic hydrocarbons and also to the polymerisation of the olefins formed. This latter reaction is a result of the tendency of the olefins to polymerise when the temperature is in the order of 500.degree. C. to 600.degree. C.; thus, in order to reduce the affects of this secondary reaction, rapid cooling (often called tempering) often has to be effected of the effluents of the reaction in order to bring them quickly from the temperature at which the pyrolysis operation was effected to a temperature which is less than about 500.degree. C., usually by means of an indirect heat exchanger.
In order to increase the selectivity of the reaction to produce olefins, studies made on the thermodynamics and kinetics of pyrolysis reactions for hydrocarbons therefore involve the following parameters:
rapid increase of the temperature of the charge to the optimum pyrolysis temperature for a given charge, and keeping that temperature as constant as possible in the reaction zone, PA0 reduction to stay time of the charge in the reactional part, PA0 reduction of partial pressure of hydrocarbon charge, PA0 rapid and effective tempering of the effluents from the reaction. PA0 a) preheating the charge which may be diluted by water vapour, PA0 b) heating that charge, or the mixture of charge and water vapour, at high temperature in tubular furnaces in order to restrict the stay time of the hydrocarbons during this pyrolysis stage, PA0 c) rapid tempering of the effluents from the reaction.
It is therefore particularly important to reduce to a minimum the contact time between the products of the reaction and the hot walls of the reactor.
Technologically speaking, these conditions have quickly led to a general process outline consisting in:
The development of thermal pyrolysis furnaces, and, in particular, of steam-cracking furnaces, has essentially been aimed at reducing the stay times and reducing the loss of charge. This has led the designers to reduce the length of the tubular reactors, and thus to increase the density of the thermal flux.
Increasing this latter can basically be done by increasing the wall temperature of the walls of the tubular reactors and/or by reducing the diameter of the tubes (which permits an increase in the s/v ratio, s being the exchange surface and v being the reactional volume).
The progress which has been made in metallurgy with special alloys which are resistant to increasingly high temperatures (INCOLOY 8OOH, HK 40, HP 40, for example) has made it possible for designers of pyrolysis furnaces, particularly those for steam-cracking, to increase the operating temperatures of these tubular furnaces, current limits in metallurgy being in the region of about 1300.degree. C.
Moreover, technology has also made progress with regard to the use of tubes of smaller diameter which are placed in parallel in order to maintain a satisfactory capacity and to remain within a suitable range of charge loss.
Proposals have also been made for several designs of pyrolysis furnaces, all of them aiming to increase the density of thermal flux at the start of the pyrolysis tube and to consequently reduce it either by using tubular reactors of increased diameter, or by grouping together at least two pyrolysis tubes to form one single one after a certain length of reaction zone (see, for example, the article by F. WALL et al published in Chemical Engineering Progress, December 1983, pages 50 to 55); non-cylindrical tubular furnaces have also been described which aim to increase the s/v ratio; thus the patent U.S. Pat. No. 3572999 describes the use of tubes of oval section, and the patent U.S. Pat. No. 3964873 describes the use of tubes of dumb-bell-shaped section.
Techniques employing thermal pyrolysis reactors, and, in particular, steam-cracking reactors have advanced from the use of horizontal tubes of approximately 100 meters (m) in length and with internal diameters in the order of 90 to 140 millimeters (mm) to the conventional use of vertically suspended tubes, approximately 40 m in length, and with diameters in the order of 60 mm which operate with residence times in the order of 0.3 to 0.4 seconds (s), and finally, the technique known as the millisecond technique, as proposed by PULLMAN-KELLOG (patent U.S. Pat. No. 3,671,198) which employs vertical, rectilinear tubes which are approx. 10 m in length, and with an internal diameter of 25-35 mm, which are brought to temperatures in the order of 1100.degree. C. (this temperature most frequently being closest to that of the limit of use of metal). The residence time of charges in this kind of furnace is in the order of 0.07 s; the loss of charge observed is in the order of 0.9 to 1.8 bar (1 bar is equal to 0.1 megapascal), and calculation of the ratio of the exchange surface s to the reactional volume v gives values in the order of 120 m.sup.-1.